One of the quality factors required for a cast product to be produced by a continuous slab casting machine is a reduced amount of inclusions entrapped in the surface layer of the cast product. Such inclusions to be entrapped in the cast product surface layer are, for example:                (1) deoxidation products occurring in a deoxidation step using aluminium and the like and suspending in molten steel;        (2) Argon gas bubbles blown into molten steel in a tundish or blown through an immersion nozzle; and        (3) inclusions occurring with mould powder sprayed on a molten steel bath surface and entrained into the molten steel as suspending substances.        
Any of these inclusions causes surface defects in steel products, so that it is important to reduce any kind of inclusions. By way of means for reducing, for example, deoxidation products and argon gas bubbles among the above described inclusions, there are popularly used processes of the type to prevent entrapment of inclusions in such a manner that intra mould molten steel is driven to move in the horizontal direction, and a molten steel velocity is thereby imparted to the surface of the molten steel to clean a solidifying surface. A practical process of applying a magnetic field for rotating the intra mould molten steel in the horizontal direction is carried out in such a manner that the magnetic field moving horizontally along the directions of long sides of the mould is driven to move in the directions opposite to each other along the opposing long side surfaces to induce a molten steel flow that behaves to rotate in the horizontal direction along the solidified surface. In this document, the application process is referred to different stirring modes, see various descriptions below, as “EMDC,” “EMDC-mode,” or “EMDC-mode magnetic field application” in combination with “EMLA,” “EMLA-mode,” “EMLA-mode magnetic field application” and/or “EMRS,” “EMRS-mode,” “EMRS-mode magnetic field application”.
The EMDC, Electro Magnetic Direct Current, braking technology, with the stirrer in a low position in the mould, is by far the most dominant technology in general and it will therefore also be possible to fix the frequency down to zero and adjust the phase angle for highest magnetic flux density in the mould. DC technology has many advantages in general, such as stability and self-regulating, i.e. if the flow velocity is higher on one side, the braking force will also be higher. In comparison with very low frequency of 1 Hz or less, DC magnetic field in the lower part of the mould can give a more stable braking control of the fluid flow in the mould.
When operating in the Electromagnetic Level Accelerating mode, EMLA, with the stirrer in a low position in the mould, the outward flow speed of the steel, towards the narrow sides, is accelerated and thereby ensuring that a dual flow pattern is achieved also for low speed casting. The optimization of the flow in the mould involves the creation of a stable two-roll flow pattern. By choosing mode and the right FC MEMS, see description below, parameters, the requested flow-pattern can be achieved at different slab geometries and casting speeds. Instead of using the analytical F-value, this can be controlled by the FC MEMS with the use of a database containing relevant parameters for different operating conditions. These parameters are usually being generated by a numerical 3D-modelling package, EM Tool, which is modelling the magnetic field, fluid flow and temperature behaviour in the mould. When operating in EMLA mode the FC MEMS should be shifted to its lower position. For low casting speeds, the FC MEMS can accelerate the fluid flow towards the narrow face in order to assure a normal flow in the mould. The F-value is converted into the molten steel surface flow velocity. However, as described in EP-A-1486274, the F-value and the molten steel flow velocity have the one- to-one relationship, so that the control can be performed by using the F-value without conversion into the molten-steel surface flow velocity.
The slab mould stirrer type FC MEMS consists of one set of stirrers per mould. Each set of stirrers consists of four linear part stirrers. The two part stirrers on each side of the mould are built together into a stirrer unit in an outer casing, and are mounted in the existing pockets behind the backup plates in the wide side water jackets. Two opposite part stirrers are connected in series and are connected to one frequency converter. Totally two frequency converters are required for one mould, and the stirrer is designed and manufactured for continuous operation in the mould. The stirrer converts the low frequency currents from the frequency converter into a low frequency magnetic field, and said magnetic field penetrates the mould copper plates and the solidified shell of the strand and induces electrical currents in the liquid steel. These currents interact with the travelling magnetic field and create forces and thus movements in the liquid steel. The stirrer comprises windings and a laminated iron core. The stirrer windings are made of copper tubes with rectangular cross section and are directly cooled from the inside by de-ionized fine water circulating in a closed loop system. The stirrer is enclosed in a protective box with sides made from non-magnetic steel sheet and the front made from non-conductive material.
Electromagnetic Rotative Stirring mode, EMRS, which is the dominating technology for stirring in a mould takes place in the upper part of the mould close to the meniscus and the position of the stirrer is of vital importance for a controlled stirring of the fluid flow. For controlled and optimum stirring it is imperative to stir at a high position in the mould and the FC MEMS must therefore be shifted upwards. Stirring in a low position will conflict with the flow exiting the nozzle and give an uncertain and turbulent flow in the mould. It is therefore proposed that the stirrer is shifted upwards with when changing from EMLA-/EMDC-mode to stirring mode. The FC MEMS generates a rotational force on the steel in the mould. The frequency converter set up allows for a lower current to be applied on the two coils where the flow is directed towards the narrow sides and thereby giving the possibility to optimize the stirring parameters. The two frequency converters, however, need to be synchronised in frequency in order to minimize possible disturbance.
An example of a similar process as described above is described in European Patent Application 1486274 (JFE Engineering Corporation) in which a EMLS, Electromagnetic Level Stabilizer, is used in combination with EMLA and/or EMRS.